Let x be the price.
The number sold is
N = 80 - 5(x -12)= 140 -5x.
The revenue is 140x -5x^2 dollars.

Try completing the square, since you probably don't know how to use differential calculus yet. Maximum revenue ($980) is at a price where
140 - 10x = 0
x = $14
At a price of $13 or $15, the revenue is $975.